Polymeric template assisted synthesis of monodisperse-porous manganese oxide microspheres: a new nanozyme with oxidase-like activity allowing biomolecule determination via bimodal sensing†
“Monodisperse-porous manganese oxide (MnOx) microspheres” that could not be obtained by current protocols were synthesized by a new template assisted method, using porous poly(methacrylic acid-co-ethylene dimethacrylate) microspheres as a template. Through the developed method, monodisperse MnOx microspheres 3.7 μm in size, with a specific surface area of 71 m2 g−1, a mean pore size of 15 nm and a porous structure covering mesopores and macropores, were obtained by the calcination of polymer/manganese oxide composite microspheres at 380 °C. A transition in the porous structure from mesoporous to macroporous form and a transition in the crystal structure from monoclinic Mn5O8 to α-MnO2 were observed with increasing calcination temperature. MnOx microspheres with appropriate size and porous properties for using them as a permeable stationary phase in a continuous microcolumn were also synthesized for the first time. The monodisperse-porous MnOx microspheres exhibited oxidase mimetic activity at neutral pH, using o-phenylenediamine (OPDA) as the substrate. The enzyme mimetic characteristics of the microspheres were also adjusted by controlling their porous properties. The decomposition of MnOx microspheres to Mn2+ ions in the presence of ascorbic acid (AA) was evaluated for bimodal sensing of this agent for the first time, which could not be achieved with MnO2 nanosheets as a frequently tried nanomaterial for biosensing applications. The particle size of the synthesized nanozyme in the micron-size range and selected substrate providing a fluorescent product allowed determination of the AA concentration via both colorimetric and fluorometric methods. The MnOx microspheres are new materials with superiorities in biosensing, column chromatography, nanocatalysis and supercapacitor applications.